In semiconductor processing, one challenge is to assure precise alignment of all the various photomasks. Mask misalignment can result in wafer destruction at worst or render certain integrated circuitry useless at best. To accommodate for mask misalignment, safeguards are built into the processing.
One place in semiconductor processing where photomasks are utilized is in defining buried or other contacts in making electrical interconnection between an inner or lower region on a substrate to an outer or higher region. One example technique and the problems associated therewith are described with reference to FIGS. 1-5. FIG. 1 illustrates a semiconductor wafer fragment indicated generally by reference numeral 10. Such is comprised of a bulk substrate region 12, field oxide 14, and a gate oxide layer 16. An initially continuous thin layer 18 of polysilicon is deposited atop field oxide 14 and gate oxide 16. Thereafter, a buried contact opening 20 is provided through layers 18 and 16 to outwardly expose substrate 12 to which electrical interconnection is to be made.
Unfortunately, the outwardly exposed portion of substrate 12 is typically oxidized to provide an undesired insulating thin layer of oxide (not shown) atop the exposed substrate. This must be removed, and is typically accomplished by a blanket HF dip. The HF dip is typically conducted after all masking material has been removed and immediately prior to deposition of any subsequent layer. Were it not for protecting polysilicon layer 18, thin gate oxide layer 16 outside of buried contact 20 would also undesirably be etched during the dip. This would be undesirable as the degradation or removal of gate oxide is not practically controllable or predictable, thus requiring protective polysilicon layer 18.
A subsequent second and thicker layer 22 of an electrically conductive material, such as conductively doped polysilicon, is deposited to make electrical connection through buried contact 20 with substrate 12. This layer is then subjected to a masking step for purposes of patterning the combined polysilicon layers 22 and 18 into conductive lines or other circuit components which integrally connect with substrate 12 at contact opening 20.
FIGS. 2 and 3 illustrate an undesired misalignment of one of the masks for producing buried contact opening 20 or the line and component produced from layers 22 and 18. Range 20 represents the buried contact mask opening, while range 24 illustrates the mask utilized for producing a line or component 26. As is apparent, the etch utilized to produce component or line 26 is misaligned relative to buried contact 20 such that an over-etch 28 into bulk substrate 12 occurs, potentially destroying or rendering the wafer useless.
To accommodate or allow for such inevitable misalignment, the target area where buried contact 20 and mask area 24 occur is enlarged, such as shown in FIGS. 4 and 5. A larger mask area range 24a is provided for producing what is commonly referred to as a buried contact enlarged cap 28. This provides for a degree of relative inevitable misalignment for assuring 100% overlap of buried contact 20 relative to desired circuit component 26 such that over-etching into the substrate is prevented.
However, this undesirably creates additional problems of its own. More wafer real estate is consumed, thus decreasing desired circuit density. Further, such typically results in an additional implant step to assure desired circuit interconnection within substrate 12. FIG. 5 illustrates desired previously provided diffusion regions 30 and 32 within bulk substrate 12. A desired implant 34 was provided through contact opening 20 prior to deposition of polysilicon layer 22. Continuous electrical interconnection among regions 32, 34 and 30 is desired. To accommodate for this, a separate buried contact implanting step to provide interconnecting implants 36 and 38 must be conducted. This adds complexity and additional steps wherein the fragile wafers might be destroyed.
It would be desirable to overcome these and perhaps other problems associated with the prior art in developing a semiconductor processing method of forming an electrical interconnection between an outer layer and an inner layer.